Method for plasma-mediated thermo-electrical ablation with low temperature electrode

ABSTRACT

Described herein are methods and apparatus for cutting a material including biological tissue. The apparatus has a cutting electrode with an elongate cutting portion. A voltage pulse waveform (typically comprising repeated bursts of minipulses) having a low or very low duty-cycle is applied to the cutting electrode to cut the tissue or other material by producing a vapor cavity around the cutting portion of the electrode and ionizing a gas inside the vapor cavity to produce a plasma. A low duty cycle cutting waveform may prevent heat accumulation in the tissue, reducing collateral thermal damage. The duration of the burst of minipulses typically ranges from 10 μs to 100 μs, and the rep rate typically ranges from 1 KHz to 10 Hz, as necessary. The apparatus and method of invention may cut biological tissue while decreasing bleeding and maintaining a very shallow zone of thermal damage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.11/784,382 filed Apr. 6, 2007. That application is herein incorporatedby reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was supported by the National Institutes of Health undercontract number RO1 EY 12888-02. The government has certain rights inthis invention.

REFERENCE TO A COMPACT DISK APPENDIX

Not applicable.

FIELD OF THE INVENTION

The present invention relates to an apparatus for cutting materialsincluding biological tissue by thermo-electrical ablation with the aidof a plasma produced around a cutting electrode and to a method fordriving such electrode with appropriate pulses.

BACKGROUND

The cutting of materials with the aid of cutting electrodes energized bya suitable power source is a known technique that is being successfullyemployed, e.g., in the field of electrosurgery. Typical electrosurgicaldevices apply an electrical potential difference or a voltage differencebetween a cutting electrode and a patient's grounded body (monopolararrangement) or between a cutting electrode and a return electrode(bipolar arrangement) to deliver electrical energy to the area wheretissue is to be cut. The voltage is applied either as a continuous trainof high frequency pulses, typically in the RF range, or as directcurrent (DC).

The prior art provides a number of exemplary designs of bipolarelectrosurgical electrodes. For example, U.S. Pat. No. 5,108,391describes a bipolar treating apparatus with a first active electrode anda second return electrode having exposed distal ends to define a bipolartip for electrosurgically treating tissue. U.S. Pat. No. 5,700,262describes a bipolar electrode with fluid channels for performingneurosurgery. Additional information about bipolar electrosurgicaldevices and knives can be found, e.g., in U.S. Pat. Nos. 4,202,337 and4,228,800 as well as numerous other open literature sources.

Depending on the conditions, the application of a voltage to a monopolarelectrode or between the cutting and return electrodes of a bipolarelectrode produces a number of physical phenomena. Most prior artdevices take advantage of one of these phenomena to perform the cut. Inparticular, one class of devices uses a gas stream that is generatedaround the cutting electrode. For example, U.S. Pat. No. 5,217,457describes an electrosurgical apparatus using a stream of gas thatshrouds the electrode and an electrosurgical apparatus incorporatingthis electrode for cutting biological tissue. U.S. Pat. No. 5,088,997also teaches the use of a stream of gas for electrosurgical proceduresfor coagulating or cutting biological tissue. On the other hand, U.S.Pat. No. 5,300,068 teaches an electrosurgical apparatus for cuttingtissue and for ablating occlusions using arc discharges produced on amonopolar electrode in response to a train of pulses. Taking advantageof a yet different phenomenon, U.S. Pat. No. 6,352,535 teaches a methodand device for electro microsurgery in a physiological liquidenvironment that uses high voltage electrical discharges ofsub-microsecond duration in a liquid medium to produce cavitationbubbles. The cavitation bubbles have a size in the sub-millimeter rangeand are used for high-speed precision cutting with an inlaid discelectrode.

In addition to taking advantage of different phenomena to perform thecut, prior art devices employ various techniques for generating andapplying the voltage to the electrode or electrodes. U.S. Pat. No.6,135,998 teaches an electrosurgical device which uses extremely shortmonopolar voltage pulses, typically shorter than 200 ns, to drive anelectrode having an inlaid disc geometry. This invention attempts tomitigate some of the negative cavitation effects, such as the damagingjets formed after the collapse of the cavitation bubble. U.S. Pat. No.5,108,391 describes a high frequency generator for tissue cutting andfor coagulating in high-frequency surgery. This device uses an electricarc discharge to perform the cutting operation. U.S. Pat. No. 6,267,757teaches a device which uses radio-frequency (RF) ablation forrevascularization. It employs a source, which delivers at least oneburst of RF energy over an interval of about 1 to about 500 ms, andpreferably about 30 to about 130 ms. This device has an elongatedinsulated, electrical conducting shaft with an uninsulated distal tip,which is configured to emit the RF energy. U.S. Pat. No. 6,364,877 alsodescribes the use of high frequency pulses applied in a continuousmanner. The teaching found in U.S. Pat. Nos. 5,697,090 and 5,766,153suggests that a continuous train of high frequency pulses can be pulsedat a rate sufficient to allow the electrode to cool.

Unfortunately, despite all the above teachings, electrosurgical methodsand apparatus generally suffer from an inability to control the depth oftissue damage (necrosis) in the tissue being treated. Mostelectrosurgical devices described above rely on a gas jet, an arcdischarge or cavitation bubbles to cut, coagulate or ablate tissue. Suchimprecise cutting methods cause tissue necrosis extending up to 1,700 μminto surrounding tissue in some cases.

In an effort to overcome at least some of the limitations ofelectrosurgery, laser apparatus have been developed for use inarthroscopic and other procedures. Lasers do not suffer from electricalshorting in conductive environments and certain types of lasers allowfor very controlled cutting with limited depth of necrosis. U.S. Pat.No. 5,785,704 provides an example of a laser used for performingstereotactic laser surgery. Unfortunately, lasers suffer fromlimitations such as slow operating speed, inability to work in liquidenvironments, high cost, inconvenient delivery systems and other defectsthat prevent their more universal application. For these reasons, itwould be desirable to provide improved apparatus and efficient methodsfor driving an electrosurgical apparatus for ablating tissue in a highlycontrolled and efficient manner while minimizing tissue damage.

The prior art has attempted to provide for more controlledelectrosurgery by relying on plasma-mediated cutting and ablation ofsoft biological tissue in conductive liquid media at low temperatures.The fundamentals of this approach, which is used predominantly in thecontinuous pulse regime and various embodiments employing it, aredescribed in the patents of Arthrocare including U.S. Pat. Nos.5,683,366; 5,697,281; 5,843,019; 5,873,855; 6,032,674; 6,102,046;6,149,620; 6,228,082; 6,254,600 and 6,355,032. The mechanism of lowtemperature ablation is called “coblation” and is described as electricfield-induced molecular breakdown of target tissue through moleculardissociation. In other words, the tissue structure is volumetricallyremoved through molecular disintegration of complex organic moleculesinto non-viable atoms and molecules, such as hydrogen, oxides of carbon,hydrocarbons and nitrogen compounds. This molecular disintegrationcompletely removes the tissue structure, as opposed to transforming thetissue material from solid form directly to a gas form, as is typicallythe case with ablation (see U.S. Pat. No. 5,683,366). More specifically,this mechanism of ablation is described as being associated with twofactors: (1) “photoablation” by UV light at 306-315 nm and visible lightat 588-590 nm produced by the plasma discharge; and (2) energeticelectrons (e.g. 4 to 5 eV) can subsequently bombard a molecule and breakits bonds, dissociating a molecule into free radicals, which thencombine into final gaseous or liquid species (see U.S. Pat. No.5,683,366). Surface temperature of tissue in this process is maintainedbetween 40-70° C. This type of ablation mechanism has low rate of tissuedissection and a very limited applicability to hard tissues such as, forexample, bones.

Despite these new advances the electrosurgical techniques are stillexperiencing a number of problems remain. First and foremost, the amountof power required to operate the prior art cutting devices remains in ahigh range of several Watts which precludes applications of thesedevices to such delicate organs as an eye. Second, the devices exhibitlarge energy and heat losses. These high losses translated intoexcessive power deposition into the tissue being ablated. Additionalheat losses to the hand piece are also substantial. Third, even the bestprior art devices operating at the lowest power levels have difficultiescutting hard biomaterials like bones and non-conducting materials suchas cellulose or plastics.

Increasingly sophisticated surgical procedures create a growing demandfor more precise and less traumatic surgical devices. For example, thecritical importance and delicate nature of the eye makes the demand forprecision and safety of intraocular microsurgical instrumentationparticularly important. For these and other reasons, it would be a majoradvance in the art to provide an apparatus and method for ablatingmaterials at low power levels. It would be particularly useful toprovide such apparatus and method that reduces heat losses to thematerial being cut as well as into the surroundings and, especially thehand piece. Furthermore, it would also be an advance to expand the rangeof materials that can be ablated to include biological tissue, celluloseand plastics.

OBJECTS AND ADVANTAGES

In view of the above shortcomings of the prior art, it is an object ofthe invention to produce a cutting apparatus and provide a method foroperating it to achieve efficient thermal ablation at low power levels,e.g., ranging down to 10 mW, in various types of materials includingbiological tissue. Specifically, it is an aim of the invention tominimize the damage zone produced during the cutting process by usingplasma-assisted cutting and minimizing heat losses into the materialbeing cut as well as the surroundings and the hand piece.

It is another object of the invention to provide a modulation format orregime for pulsed operation of the cutting apparatus to minimize adverseeffects in cutting biological tissue. In particular, it is one object ofthe invention to provide an electrosurgical cutting pulsing regime (alsoreferred to as a pulsed waveform) that operates at low and very lowduty-cycles.

It is yet another object of the invention to reduce the averagetemperature that the cutting apparatus acting on the biological tissueoperates on during cutting.

Yet another object of the invention is to provide a versatile cuttingelectrode geometry for efficient cutting and removal of material. Someof these electrodes and the driving waveforms are specifically designedfor applications specific types of surgery, including IMA and radialartery access and harvesting (cardiac surgeries), arthroscopy (sportsmedicine procedures); tonsillectomy (ENT surgeries); LEEP, abdominalhysterectomy (Ob/Gyn procedures); fusion, microdiscectomy (Neuro/Spinalprocedures); intra and extra ocular procedures (Ophthalmology).

These and other objects and advantages will become apparent upon reviewof the following description and figures.

BRIEF SUMMARY OF THE INVENTION

The objects and advantages of the invention are achieved by a method forcutting a material including conducting and non-conducting materialssuch as biological tissue, cellulose or plastic. During cutting thematerial may be submerged in a conductive liquid medium. Dry cutting mayalso be performed. The method may involve providing a cutting electrodewith an elongate cutting portion and a return electrode. The elongatecutting portion may have an aspect ratio of length to width larger than1 and preferably larger than 5. A thin cutting electrode may allow fordissection of tissue with low energy deposition. A voltage is typicallyapplied between the two electrodes so that the cutting electrode isheated to produce a vapor cavity around the elongate cutting portion andgas inside the vapor cavity is ionized to produce a plasma. The plasmamay maintain the electrical conductivity between the electrodes. Thevoltage applied between the electrodes may be modulated in pulses havinga modulation format selected to minimize the size of the vapor cavity,the rate of formation of the vapor cavity and heat diffusion into thematerial as the material is cut with an edge of the elongate cuttingportion of the cutting electrode. The modulation format may becharacterized as a low (or very low) duty-cycle pulse waveform.

The modulation format may include pulses having a pulse duration in therange from 10 μs to 10 ms. The pulses may be composed of minipulseshaving a minipulse duration in the range between 0.01 μs and 10 μs andan interval ranging from 0 to 10 μs between the minipulses. Theminipulse duration may be selected from the range substantially between0.2 and 5 μs and the interval between them is shorter than a lifetime ofthe vapor cavity. The peak power of the minipulses can be varied fromminipulse to minipulse.

When the method is used for cutting biological tissue it may bepreferable to use minipulses with alternating polarity, includingbipolar minipulses, in which a minipulse has both negative and positivecomponents. Thus, the modulation format may contain minipulses thatexhibit alternating positive and negative polarities or both positiveand negative polarities per pulse. This modulation format may limit theamount of charge transfer to the tissue and reduce or greatly minimizevarious adverse tissue reactions such as muscle contractions andelectroporation. Additional devices for preventing charge transfer tothe biological tissue may be employed in combination with thismodulation format or separately when the method of invention is appliedin performing electrosurgery.

In the same or in an alternative method of the invention the minipulsesmay be further made up of micropulses. When the modulation formatincludes micropulses it is preferred that they have a duration rangingbetween 0.1 and 1 μs.

It is well-known that spark discharges develop in advance of an arcdischarge. In accordance with the invention it is preferable to adjustthe modulation format to permit spark discharges while preventing arcdischarges. For example, the modulation format such as minipulseduration and peak power may be adjusted to permit spark discharges whileavoiding arc discharges. Furthermore, the voltage and the modulationformat may be selected such that the temperature of the elongate cuttingportion of the cutting electrode and of the plasma are maintainedsignificantly above the boiling temperature of water. Preferably, thetemperature of the elongate cutting portion is maintained between about100 and 1,000° C. This temperature may be a peak temperature for thecutting electrode. The average (e.g., RMS) temperature may besubstantially less than 100 ° C. For example, the average temperature ofthe cutting electrode (e.g., the exposed electrode surface of thecutting electrode) may be less than approximately 50° C. In somevariations, the average temperature of the exposed cutting electrodesurface is less than 40° C. (e.g., approximately 37 ° C.).

The apparatus and systems described herein may be equipped with thecutting electrode with the elongate cutting portion and a returnelectrode. A voltage source may be used for applying the voltage betweenthe cutting and return electrodes to produce the vapor cavity withplasma. A pulse control may be provided for controlling the modulationformat of the voltage applied between the electrodes. The pulse controlmay include a peak power control and a duration control for adjustingpulse power, pulse duration and pulse interval. Additional controls maybe provided to control the pulsing waveform, including a control forrepetition rate (e.g., repetition of bursts of minipulses), minipulseburst duration control, minipulse duration control, number of minipulsesper burst, and voltage range applied (e.g., ±peak voltage). The pulsecontrol may be limited to prevent application of a duty cycle above athreshold during a cutting pulsing waveform. For example the rangesprovided to adjust any of the parameters given above may be limited sothat the duty cycle of the pulsing waveform is below a certain value(e.g., 10%, 7%, 5%, 2.5%, 1%, etc.).

The shape, size, and length of the cutting electrode and the elongatecutting portion can vary according to the material being cut. For anumber of electrosurgical applications the elongate cutting portionshould have a width (or thickness) of between about 1 μm and 200 μm andpreferably between 10 μm and 100 μm. The elongate cutting portion canhave various cross sections including circular, e.g., it is in the formof a wire. In some variations, the entire cutting electrode can be inthe form of a wire electrode. In some variations, a blade or edgedcutting electrode (including an insulated region and an uninsulatededge) may be used. Examples of cutting electrode may be found inapplication including U.S. Ser. No. 10/779,529, filed Feb. 13, 2004(titled “Electrosurgcial System with Uniformly Enhanced Electric Fieldand Minimal Collateral Damage”), herein incorporated by reference in itsentirety. The elongate cutting portion can have one or more bends orcurves. For example, in certain electrosurgical applications theelongate cutting portion can be L-shaped or U-shaped. In someembodiments the elongate cutting portion can form a loop, e.g., it canbe a looped wire electrode. In some embodiments it is advantageous toprovide a device for advancing the wire electrode such that a length ofthe wire used for cutting can be adjusted during the application, whenrequired. Such adjustment affects the impedance of the electrode and canbe used for control of power dissipation. In addition, a fresh portionof the wire can be extended to replace the eroded portion. In oneparticular embodiment, the elongate cutting portion and the terminalportion of return electrode are both shaped into a shape suitable forcapsulotomy.

In embodiments where transferring charge to the material should beavoided, e.g., when the material being cut is biological tissue, theapparatus or systems described herein may include a device forpreventing charge transfer through the non-conducting material. Forexample, a circuit with a separating capacitor, e.g., an RC-circuit, canbe used for this purpose.

The devices, system and methods described herein operate using a pulsewaveform that has a low (to very low) duty cycle. This is unlike priorart electrosurgical cutting systems, which typically operate withcontinuous (e.g., between 100% to 50% duty cycle). For example, intraditional RF cutting, cutting is achieved by the accumulation ofaction over long time (i.e., long duty cycle). In continuous RF systemsthe duty cycle is approximately 1 (or 100%) during cutting. Other pulsedcutting waveforms may have slightly lower duty cycle, typically betweenapproximately 0.5 (or 50%) and 0.3 or (30%). In contrast, the pulsewaveforms described herein operate at a very low duty cycle (e.g.,typically less than or equal to 0.1 or 10%.

Thus, methods for thermo-electrical cutting of biological tissue aredescribed herein, including methods for the application of a lowduty-cycle pulse waveform to a cutting electrode, wherein the cuttingelectrode is connected to a voltage control unit. Material (including abiological material) may then be cut with the cutting electrode duringapplication of the low duty-cycle pulse waveform.

The low duty-cycle pulse waveform may include pulsing waveformsinvolving the application of a plurality of pulses having a duty cycleof less than about 10%, less than about 5%, and duty cycles of betweenabout 2.5% and about 0.01%.

In some variations, the low duty-cycle cutting waveform comprises aplurality of pulses wherein each pulse comprises a burst of minipulses.Each burst of minipulses that make up a “pulse” is repeated with arepetition rate. For example, the repetition rate may be selected frombetween about 10 Hz and about 500 Hz, so that the time between bursts ofminipulses (the “interburst repetition rate”) may vary between 100 msand 2 ms. The burst of minipulses typically have a duration betweenabout 10 μs and 100 μs.

A burst of minipulses typically includes a plurality of minipulses(e.g., two or more minipulses per burst). Any appropriate number ofpulses may be present, depending on the duration of each minipulse andthe duration of the burst of minipulses. In some variations, eachminipulse within the burst of minipulses has a duration of between about10 ns to about 100 μs. The minipulses within the burst of minipulses maybe bipolar minipulses. For example, alternating minipulses may havedifferent polarities. In some variations, each individual minipulse isbipolar, and has a positive and a negative voltage component. Theminipulses within the burst may be separated by a minipulse intraburstinterval. For example, between 0 and 100 μs. In some variations, theminipulses within each burst of minipulses are continuously applied.

The voltage of the pulses (e.g., the minipulses) within a low duty-cyclecutting waveform may have any appropriate voltage. In variations inwhich the minipulses are bipolar, the voltage may vary between about−600 V and about +600 V, or between about −500 V and +500 V, or betweenabout −400 V and +400 V.

As a consequence of the low duty-cycle activation of the cuttingelectrode, material heated during the pulse cools down between thepulses, thus the average temperature of the cutting electrode duringcutting is typically less than 100° C., and may be less than 50° C., orless than 40 ° C. During application of a low duty-cycle pulse waveform,a plasma may be transiently formed along the edge of the cuttingelectrode. For example, a vapor cavity may be formed during the burst ofminipulses, and the vapor cavity may be ionized. During the periodbetween the bursts of minipulses, the temperature of the cuttingelectrode may cool or relax back down, so that the average temperatureof the cutting electrode during cutting is much lower than the peaktemperature during plasma formation. However, the peak temperature ofthe cutting electrode during cutting (e.g., along the cutting edge atthe formation of plasma) may be greater than 100 ° C.

Described herein are methods for thermo-electrical cutting of biologicaltissue including the steps of applying a pulse waveform having aduty-cycle of less than 10% to a cutting electrode (wherein the pulsewaveform comprises a plurality of bursts of minipulses having aninterburst repetition rate of between about 10 Hz and 500 Hz, and aminipulse burst duration of between about 5 μs and about 200 μs or about10 μs and about 100 μs), and cutting the tissue with the cuttingelectrode during application of the pulse waveform.

Also described herein are methods for thermo-electrical cutting ofbiological tissue including the steps of forming a plasma on a cuttingelectrode by applying a low duty-cycle pulse waveform (wherein the pulsewaveform comprises a plurality of bursts of minipulses having aninterburst repetition rate of between about 10 Hz and 500 Hz, and aminipulse burst duration of between about 5 μs and about 200 μs orbetween about 10 μs and about 100 μs), and cutting the tissue with thecutting electrode during application of the pulse waveform.

Also described herein are methods for thermo-electrical cutting ofbiological tissue including the steps of applying a low duty-cycle pulsewaveform to a cutting electrode wherein the cutting electrode has peaktemperature during application of the pulse waveform of greater than100° C. and an average temperature during application of the pulsewaveform of less than about 50° C. The tissue may then be cut with thecutting electrode during application of the low duty-cycle pulsewaveform. In some variations, the average temperature during applicationof the pulse waveform is less than about 40° C.

Also described herein are methods of simultaneously cutting andhematostais of a biological tissue. Thus, the tissue may be cut in a lowduty-cycle pulse waveform that also results in constriction of thevessels adjacent to the cut and thereby decreases the blood flow fromthe cut tissue. The method may therefore include the steps of contactinga biological tissue with a cutting electrode, applying a low duty-cyclepulse waveform to the cutting electrode (wherein the cutting electrodehas an average temperature during application of the pulse waveform ofless than about 50° C.), and cutting the tissue with the cuttingelectrode during application of the low duty-cycle pulse waveform whileat least partially constricting the blood vessels adjacent to the cuttissue by application of the low duty-cycle pulse waveform to thecutting electrode.

As described above, the low duty-cycle cutting waveform may involveapplying a plurality of pulses having a duty cycle of less than about10%, less than about 5%, or less than about 2.5%. The low duty-cyclecutting waveform may be a pulse waveform having a plurality of pulseswherein each pulse is made up of a burst of minipulses. The burst ofminipulses may be bipolar minipulses. In some variations, the durationof each minipulse within the burst of minipulses is between about 10 nsand about 100 μs. In some variations, the minipulses within each burstof minipulses are continuously applied, while in other variations, thereis a delay between minipulses within the burst. In some variations, thelow duty-cycle cutting waveform consists of a plurality of bursts ofminipulses having an interburst repetition rate of between about 10 Hzand 500 Hz, and a minipulse burst duration of between about 5 μs andabout 200 μs. In some variations, the voltage of the pulses within thelow duty-cycle cutting waveform may be between about −500 V and about+500 V. Also, the average temperature during application of the low-dutypulse waveform may be less than about 40° C. In some variations, thecutting electrode has a peak temperature during application of thelow-duty pulse waveform of greater than about 100° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a three-dimensional view of an apparatus according to theinvention employed in cutting biological tissue.

FIG. 2 is a graph illustrating a pulse modulation format according tothe invention.

FIG. 3 is a graph indicating the qualitative dependence of thecavitation bubble diameter and heat diffusion on duration of the pulse.

FIG. 4A is a graph illustrating the conversion of electrical energy ofthe discharge (1 mJ) into the mechanical energy of the bubble measuredas a function of pulse duration for the apparatus of FIG. 1.

FIG. 4B is a graph illustrating the cavitation bubble size, energydeposition and heat diffusion as a function of pulse duration for theapparatus of FIG. 1.

FIG. 5 is a photograph of the use of cutting electrode with elongatecutting portion for cutting paper.

FIG. 6 is a graph of a pre-pulse and post-pulse used in accordance withthe invention.

FIG. 7 is a graph illustrating the use of micropulses in accordance withthe invention.

FIG. 8 is a graph illustrating the use of minipulses of alternatingpolarity in accordance with the invention.

FIG. 9 illustrates an apparatus of the invention used in cutting amaterial.

FIG. 10 illustrates an apparatus of the invention using a shaped cuttingelectrode.

FIGS. 11A-C are partial views of alternative embodiments in accordancewith the invention.

FIG. 12 illustrates an apparatus of the invention designed forcapsulotomy.

FIGS. 13A-13E illustrate various low duty-cycle pulse regimes(waveforms).

FIGS. 14A-14C illustrate heat patterns of cutting electrodes underdifferent pulsing regimes.

FIGS. 15A and 15B are sections through tissue cut using a cuttingelectrode and a low duty-cycle pulse waveform.

FIG. 16A is a graph comparing bleeding using different cuttingparadigms, as described herein.

FIG. 16B is a graph illustrating wound strength for cuts made bydifferent cutting paradigms, as described herein.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an apparatus 10 for cutting a material 12 submergedin a conducting liquid medium 14. In this embodiment material 12 is abiological tissue made up of various types of tissue including muscletissue 12A, nerve tissue 12B, bone 12C and soft tissue 12D. In general,however, material 12 can be any conducting or non-conducting materialwhich requires cutting and can include materials such as cellulose,e.g., wood and cellulose-based materials as well as various types ofnon-conducting plastics. Liquid medium 14 can be any type ofelectrolyte. In the present example, liquid medium 14 is a physiologicalmedium, for example an isotonic saline solution.

Apparatus 10 has a cutting electrode 16 with an elongate cutting portion18. In this embodiment, the entire cutting electrode 16 is in the formof a wire electrode with circular cross section defined by a radiusr_(e). The material of wire electrode 16 can be any suitable conductorsuch as a metal like Tungsten, Titanium, Molybdenum, etc. or an alloy.In the present embodiment electrode 16 is made of Tungsten wire. Cuttingelectrode 16 is surrounded by an insulating layer 20 and a returnelectrode 22. Insulating layer 20 can be any dielectric material orcombination of materials such as ceramic, plastic, glass, and/or airthat provide electrical insulation between electrodes 16 and 22.Electrodes 16 and 22 are arranged coaxially along a center line 26.Cutting portion 18 protrudes beyond insulating layer 20 and returnelectrode 22. In fact, a length L of elongate cutting portion 18 isexposed. The aspect ratio of length L to width w (w=2r_(e)) of cuttingportion 18 is at least 1 and preferably more than 5.

A voltage control unit 24 is connected to cutting electrode 16 and toreturn electrode 22. Voltage control unit 24 has a voltage generator forproducing a voltage to be applied between electrodes 16, 22. Unit 24also has a pulse control for pulsing the voltage in accordance with apredetermined modulation format (pulse regime or pulse waveform), asdescribed below. The pulse control has a peak power control and aduration control for adjusting a pulse power, a pulse duration τ and apulse interval.

The application of a pulse waveform to the cutting electrode may resultin the formation of a thin layer of a plasma 28 around elongate cuttingportion 18. To achieve this, electrodes 16, 22 of apparatus 10 may beimmersed in conductive medium 14 where tissue 12 is submerged and avoltage is applied between electrodes 16, 22 such that medium 14 isheated to produce a vapor cavity 30 around cutting portion 18. Duringheating, an amount of medium 14 is vaporized to produce a gas 32 insidevapor cavity 30. In the present case medium 14 is saline and thus gas 32is composed predominantly of water vapor, a small amount of oxygen andhydrogen and trace amounts of NaCl. The layer of gas 32 is ionized inthe strong electric field around cutting electrode 16 to make up thethin layer of plasma 28. Because plasma 28 is electrically conductive,it maintains electrical conductivity between electrodes 16, 22. Althoughmany of the examples described herein include the use of a conductivemedium, it is to be understood that many of the devices, systems andmethods described herein may be used even in the absence of a conductivemedium.

As described herein, a cutting electrode may be any electrode with arelatively narrow exposed region so that the vapor cavity and ionizationcan efficiently occur around the edge. For example, the cuttingelectrode may have an exposed region that is less than 50 μm thick(e.g., from the edge of an insulation region) and a length, L (where Lcan be any appropriate length, such as X mm). In some variations, thecutting electrode is a wire that is less than 100 μm in diameter.

In some variations, the pulse waveform is matched to the cuttingelectrode. For example, the cutting electrode may have a length, L andan exposed (uninsulated) cross-sectional area through the length that isless than 100 μm, or less than 50 μm, or less than 20 μm, or less than10 μm. In general, cutting electrodes may have a long and narrow exposedelongated length, although point cutting electrodes (that are notparticularly elongated) may also be used.

In contrast to the prior art, it is important that the size and rate offormation of vapor cavity 30 as well as heat diffusion into tissue 12 beminimized. The size and rate of formation of cavity 30 are related andcan be minimized by modulating the voltage applied between electrodes16, 22 by the pulse control of unit 24 in accordance with a modulationformat, which may also be referred to as pulse waveform. Specifically, apulse control may modulate the applied voltage in pulses 34, as shown inFIG. 2. The modulation format of pulses 34 is selected to minimize thesize of vapor cavity 30, the rate of formation of vapor cavity 30 andalso heat diffusion into tissue 12.

To better understand the principles behind selecting the modulationformat to achieve this minimization we now refer to the qualitativegraphs in FIG. 3. Graph 38 illustrates the radius around elongatecutting portion 18 to which heat diffuses as a function of duration τ ofpulse 34. As duration τ of pulse 34 increases heat diffuses deeper intotissue 12. This diffusion of heat causes thermal damage to tissue 12 andit is to be avoided. It should be noted, that the application of a longtrain of very high frequency pulses, e.g., RF pulses, will effectivelyact as one long pulse whose duration is equal to the entire duration ofthe pulse train. Hence, prior art devices operating in the continuouswaveform and applying RF pulses (see Background section) suffer fromhigh heat diffusion and consequently cause large thermal damage tosurrounding tissue.

Graph 40 illustrates the maximal radius of vapor cavity in this casealso referred to as bubble 30 (see FIG. 1), or cavitation bubble, whichis formed at constant pulse energy around cutting electrode 16. Now, theradius of cavitation bubble 30 initially increases with increasing pulseduration τ and then decreases and approaches zero as duration τ of pulse34 tends to infinity (continuous current). Graphs 38 and 40 intersect ata pulse duration τ_(c) at which heat diffusion is still relativelyinsignificant while the radius of bubble 30 is already small enough notto cause significant tissue damage. Thus, by choosing duration τ ofpulse 34 in a range 42 around τ_(c) heat damage and mechanical damagedue to cavitation bubble 30 are minimized. In fact, choosing duration τof pulses 34 so as not to produce large cavitation bubble 30 isequivalent to minimizing the size and rate of formation of vapor cavity30. A person skilled in the art will appreciate that the exact shape ofgraphs 38, 40 and range 42 will vary depending on specific parameterssuch the exact composition of tissue 12, salinity of electrolyte 14 andgeometry of electrode 16.

FIG. 4A shows a graph 44 of the conversion of the electrical energy ofthe discharge for a discharge energy equal to 1 mJ and electrode 16diameter of 25 μm into mechanical energy of bubble 30 measured as afunction of duration τ of pulse 34. Efficiency of the conversiondecreases with increasing duration τ of pulse 34 once pulse 34 is longerthan about 3 μs. In FIG. 4B the radius of single bubble 30 isillustrated by graph 38′ as a function of pulse duration τ in a 1 mJdischarge. At pulse duration τ above 50 μs a sequence of bubbles isformed with maximal radii reducing with increasing duration τ, asdepicted by separate rhombuses. Graph 48 represents the penetrationdepth into material 12 of electric field E(r), here equal to radiusr_(e) of cutting portion 18. Graph 40′ represents the radius aroundcutting portion 18 to which heat diffuses assuming constant temperatureof cutting portion 18 in one dimensional geometry.

A range 42′ in which pulse duration τ is optimized and in which bothcavitation and heat diffusion are comparable with field penetrationdepth is between 50 μs and 2 ms. Under different conditions range 42′will vary, but optimal duration τ of pulses 34 will generally fallbetween 10 μs and 10 ms. In this range 42′ the size and rate offormation of vapor cavity 30 as well as heat diffusion into tissue 12are minimized. The thermal damage zone in tissue 12 due to heatdiffusion is dependent mostly on duration τ of pulse 34. Specifically,varying duration τ of pulses 34 between 0.1 and 100 ms changes the depthof the heated zone in tissue 12 between 10 and 300 μm ranging fromsingle cellular layer with no hemostatic effect to complete hemostasisin most of tissues 12A, 12B, 12C and 12D.

In the exemplary pulse waveform shown in FIG. 2, pulses 34 can bedelivered in various modulation formats including continuous pulses orbursts of short pulses or minipulses 50. Thus, a pulse may be a burst ofminipulses having a duration for the burst of minipulses equal to τ.Pulses 34 may be separated by a separation 49 of at least 1 ms, andpreferably at least 2 ms while pulses 34 themselves are composed of anumber of minipulses 50, as shown. The amplitude and duration ofminipulses 50 determine the spatial extent and density of plasma 28. Toavoid excessive overheating of tissue 12 the modulation format isadjusted so that plasma 28 is maintained at the regime of streamer andspark discharges but the arc discharges are prevented. Specifically,duration and peak power of minipulses 50 are adjusted to permit sparkdischarges and to prevent arc discharges. In most cases, limiting theduration of minipulses 50 to less than several μs will accomplish thisgoal. In fact, the duration of minipulses 50 may be kept in the rangebetween 10 ns and 100 μs and preferably between 0.2 and 5 μs. Theinterval between minipulses 50 is preferably selected in the rangebetween 0 (e.g., continuous) and 10 μs. Such short times are sufficientfor ionization and development of the spark discharges but not forcreation of the arc discharge.

An arc discharge is a highly luminous and intensely hot discharge ofelectricity between two electrodes, in this case between electrode 16,and more precisely its cutting portion 18, and return electrode 22. Thearc discharge is initiated when a strong electric forces draw electronsfrom one electrode to the other, initiating the arc. It is typically acontinuous discharge characterized by high current and low voltageacross the arc. On the other hand, a spark discharge has a high voltageand short duration.

If the intervals between minipulses 50 do not exceed a lifetime of vaporcavity 30, the ionization will be maintained by minipulses 50 untilvapor cavity 30 collapses. Hence, in any situation, the intervalsbetween minipulses 50 should be kept shorter than the lifetime of vaporcavity 30. For example, the lifetime of a 100 μm wide vapor cavity 30 isabout 10 μs, thus minipulses 50 should be delivered at intervals notlonger than 10 μs when working with such cavity width.

In contrast to prior art devices, apparatus 10 cuts tissue 12 using aside or edge of cutting portion 18, i.e., the entire length L of cuttingportion 18 is available for performing the cut. Rapid and efficientablation of tissue 12 may be achieved when the temperature of cuttingportion 18 and layer of plasma 28 around it are maintained significantlyabove the boiling temperature of water. In some variations, thetemperature is efficiently maintained by having the cutting portion 18be long and thin, i.e., having a small radius—a few tens of microns—andan aspect ratio (length to width) of at least 1 and preferably at least5. Such thin cutting portions 18 also reduce the amount of heat flowthrough the metal back into a hand piece (not shown).

In fact, heat flow W through cutting portion 18 in this example is equalto:

W=XΔTS/L

where S=πd²/4 is the cross section area of cutting portion 18. In theabove equation X is the coefficient of thermal conductivity and ΔT isthe difference in temperature between the hot and cold parts of wireelectrode 16, L is the length of cutting portion 18 and d=2r_(e).Evaporation rate of tissue 12 is equal to:

V=Ldv

where v is the velocity of advance of cutting portion 18 through tissue12. The amount of power deposited in tissue 12 to achieve suchevaporation rate is:

P=V·ρ(CΔT ₁+δ)

where ρ is the density of tissue 12, C is its heat capacity, ΔT₁ is thetemperature rise from ambient to 100° C., and δ is the specific heat ofevaporation (for tissue mostly composed of water the specific heat ofevaporation of water δ=2.26×10³ J/g can be used in the calculation). Toprevent cooling of cutting portion 18 and of layer of plasma 28 by heattransfer via electrode 16, power deposition P should be keptsignificantly larger than the heat flow W, i.e., P>>W. In the presentexample, electrode 16 is made of Tungsten which has a heat conductivityX=178 W/m*K, ΔT₁=70° K. and cutting portion 18 is advanced throughtissue 12. For example, at ΔT=250° K. and v=1 mm/s one obtains thecondition L²/d>>14 mm from the above equations. Therefore, toefficiently prevent cooling when cutting portion 18 has a length L=1 mmthe diameter d=2r_(e) of cutting portion 18 should be less than 70 μm.For ΔT=70° K. and the rest of the parameters remaining the same we willobtain the conditions L²/d>>4 mm This means that a 1 mm long cuttingportion 18 should not be thicker than 250 microns.

In some variations, the temperature of cutting portion 18 can bemaintained as low as about 100° C., but it may be much higher, rangingup to 1000° C. In this temperature range tissue 12 is rapidly evaporatedand thus ablated. Due to the turbulent flow of liquid boiling at theedges of vapor cavity 30 the interface with tissue 12 is only minimallyoverheated and damaged.

In the regime of heating produced by plasma 28 the temperature ofcutting portion 18 may be stabilized by a naturally occurring negativefeedback mechanism as follows. In the areas where the vapor sheet ofcavity 30 becomes thinner, the electric impedance is reduced and thusmore current flows. The increased current results in increasedgeneration of Joule heat in that area, and thus more electrolyte 14 isevaporated thereby increasing the thickness of vapor cavity 30 in thatarea. This mechanism stabilizes the thickness of vapor cavity 30 aroundcutting portion 18 and the thermal conditions of cutting portion 18.When tissue 12 is brought into ionized vapor cavity 30, thus reducingits thickness in that area, more current flows into tissue 12 than intoplasma 28, since the impedance of tissue (which is typically similar tothat of electrolyte 14) is much lower than that of plasma 28. Thus, moreheat is generated in the area where tissue 12 is positioned inside vaporcavity 30.

Application of thin elongated electrode (for example a wire electrode)may allow for minimization of the amount of material evaporated duringtissue dissection as well as for minimization of the depth of the damagezone produced at the edges of the cut, as shown below. In the presentembodiment, the electric field E(r) around cylindrical cutting portion18 is reciprocal to the distance from it, and the density of Joule heatgenerated in liquid by the discharge is reciprocal to the square of thatdistance. Thus, thinner cutting portion 18 results in a more confinedenergy deposition. In fact, the electric field E(r) around cylindricalcutting portion 18 scales with distance r as follows:

$E = \frac{\left( {E_{e}r_{e}} \right)}{r}$

where E_(e) is the value of the electric field on the surface of cuttingpotion 18. Thus, the difference in voltage on the surface of cuttingportion 18 and at a distance R from electrode 16 is:

U _(e) −U _(R)=∫^(r) _(R) E(r)dr=E _(e) r _(e) (ln R−ln r _(e)).

The electric field becomes spherical at distances larger than length Lof cutting portion 18, and thus it can be assumed that the electricpotential drops to zero for distances larger than L. Therefore, theelectric field E_(e) at the surface of cutting portion 18 is:

$E_{e} = {\frac{U_{e}}{r_{e}\left( {{\ln \; L} - {\ln \; r_{e}}} \right)}.}$

The power density w of the Joule heat generated in electrolyte 14 isthen:

${w = {{j^{2}\gamma} = {\frac{E_{e}^{2}}{\gamma} = \frac{U_{e}^{2}}{{r_{e}^{2}\left( {{\ln \; L} - {\ln \; r_{e}}} \right)}^{2}\gamma}}}},$

where j is the current density and y is the resistivity of electrolyte14. The minimal energy density for overheating of the surface layer ofelectrolyte 14 (assumed to be water) by pulse 34 of duration τ is:

A=w·τ=ρ·C·ΔT

where ΔT is the total temperature rise in the surface layer ofelectrolyte 14 during pulse 34, ρ is the density of water and C is itsheat capacity. Therefore, the voltage U required for initiation ofvaporization during pulse 34 of duration τ is:

U=r _(e)(lnL−ln r _(e))√{square root over (ρ·C·ΔT·γ/τ)}.

The voltage U and associated energy deposition can be reduced bydecreasing the radius r_(e) of cutting portion 18. In general, ambienttemperature is about 30° C. when operating in biological tissue 12 of alive subject, boiling temperature is 100° C., ρ=1 g/cm^(3, C=)4.2J/(g·K) and γ˜70 Ohm·cm. With these values we obtain A˜300 J/cm³ andU=260 V for pulse 34 of duration τ=0.1 ms, r_(e)=25 μm and L=1 mm.

Since the electric field is reciprocal to the distance from thecylindrical electrode, the field efficiently penetrates into theelectrolyte to the depth similar to the radius of the electrode. Thisminimal amount of energy required for creation of the vapor cavityaround the electrode is:

A=w·τ=ρ·C·ΔT·π·d ² ·L,

where d is the diameter of the electrode. Minimal depth of the damagezone at the edges of the cut will thus be similar to the radius of theelectrode. Thus, reduction in radius of the electrode results inreduction in the power consumption and in the width of the damage zoneproduced at the edges of the cut. The threshold voltage U_(th) requiredfor reaching the threshold electric field E_(th) to ionize gas 32 andproduce plasma 28 is:

U _(th) =E _(th) r _(e) ln(R/r _(e)),

where R is the radius of vapor cavity 30, as shown in FIG. 1. Thresholdvoltage U_(th) can be decreased by reducing radius r_(e) of cuttingportion 18. This also results in a lower power dissipation andconsequently in a smaller damage zone in tissue 12.

Vapor cavity 30 filled with plasma 28 and surrounding cutting portion 18of cutting electrode 16 serves three major functions. First, itthermally isolates cutting electrode 16 from electrolyte 14 thusallowing for efficient heating. Second, the electric impedance of plasma28 is much higher than that of tissue 12, thus Joule heating isgenerated mostly in plasma 28 and not in the surrounding liquidenvironment. Third, since both electrical and thermal conductivity oftissue 12 is much higher than that of a vapor (gas 32), when tissue 12is introduced inside vapor cavity 30 with plasma 28 it attracts bothelectric current and heat flow, which results in fast overheating andevaporation.

Another advantage of the cylindrical geometry of cutting electrode 16 ascompared to prior art point sources (inlaid disc geometry) is that itallows for cutting tissue 12 with the side edge of cutting portion 18.Prior art point sources (see U.S. Pat. No. 6,135,998) produce a seriesof perforations when a train of pulses is applied. These perforations donot always form a continuous cut leaving behind bridges between theedges of the cut. To dissect these bridges the secondary scans arerequired and targeting these thin and often transparent straps of tissueis very difficult and time consuming Cylindrical cutting portion 18solves this problem by enabling the cutting by its edge and not only byits end or tip.

In order to reduce unnecessary energy deposition, e.g., duringelectrosurgery, the voltage of source 24 can be set to a level which issufficient for ionization of only a thin layer of vapor. Thus, in areaswhere vapor cavity 30 is too large (typically above several tens ofmicrons) no plasma 28 will be formed. As a result, ionization andformation of plasma 28 will only take place in the areas of proximity orcontact between generally conductive tissue 12 and conductive cuttingportion 18. In other parts of vapor cavity 30 gas 32 will not be ionizedand thus it will electrically insulate cutting electrode 18 preventingheat deposition into the surrounding environment. FIG. 5 illustratescutting electrode 16 with cutting portion 18 of radius r_(e)25 μmimmersed in isotonic saline solution touching the edge of a material 52.This figure shows clearly the formation of plasma 28 at the point ofcontact with material 52. In this case material 52 is made of celluloseand is in fact a sheet of paper. Cutting portion 18 is touching an edgeof material 52 that is about 250 μm thick. As is clearly seen, plasma 28is generated only in the area of contact between cutting electrode 18and paper 52.

Although the cutting electrodes described above (including the derivedequations) are predominantly wire electrodes, other cutting electrodesmay be used. In particular, blade or knife-type cutting electrodes maybe used, in which only the cutting edge (e.g., an exposed edge region isuninsulated).

To further reduce the energy deposition, cavity 30 can be created byelectrochemical generation of gas 32, i.e., by electrolysis of water,rather than by its vaporization. For this purpose the pulse control andsource 24 can vary the voltage between parts of the pulse or evenbetween two successive pulses, as shown in FIG. 6. First, source 24applies a pre-pulse 54 of relatively low voltage. This low voltageshould be sufficient for electrolysis and can be in the range of severaltens of Volts. In accordance with well-known principles, the applicationof such low voltage will yield oxygen gas on the anode and hydrogen gason the cathode. The user can choose whether to use oxygen or hydrogen asgas 32 by selecting the polarity of pre-pulse 54, such that cuttingportion 18 is either the anode or cathode. It should be noted, thatapplying a pulse composed of minipulses with alternating polarity (seeFIG. 8 and below description) will generate a mixture of oxygen andhydrogen.

Next, pulse control and source 24 increases the voltage to a relativelyhigh level in a post-pulse 56. The voltage of post pulse 56 can be inthe range of several hundred Volts to complete the formation of vaporcavity 30 and to ionize gas 32 to form plasma 28. A sequence ofcombination pulses containing pre-pulse 54 and post-pulse 56 can be usedto drive apparatus 10. Alternatively, a single combination pulse can befollowed by a series of regular pulses 34 composed of minipulses 50, asdescribed above. Embodiments of the method taking advantage ofelectrochemical generation of gas 32 around cutting portion 18 ofelectrode 16 obtain a substantial pulse energy reduction.

The rate of evaporation of electrolyte 14 may depend on its temperature.There is always a delay between the moment when electrolyte 14 reachesboiling temperature (boiling temperature of water) and the moment whenformation of vapor cavity 30 disconnects the current flowing throughelectrolyte 14 between electrodes 16, 22. When vapor cavity 30 forms,gas 32 stops the current flow and prevents further heating. Just beforethis time an additional energy is deposited that leads to overheating ofelectrolyte 14 and thus to explosive (accelerated) vaporization. Thiseffect results in formation of a larger vapor cavity 30 and turbulencearound cutting portion 18 of electrode 16. To prevent such overheatingthe energy for initial boiling should be delivered at a lower voltage,but as soon as vapor cavity 30 is formed, the voltage should beincreased to achieve fast ionization of gas 32. Several solutions can beemployed to address this problem.

In accordance with a first solution, a low impedance line 58, asindicated in dashed line in FIG. 1, is used instead of a standardelectrical connection between the output of pulse generator in unit 24and cutting electrode 16. In accordance to well-known principles, lowimpedance line 58 will cause the rising edge of a pulse to be reflectedfrom the output end if the output impedance is high. This conditionoccurs when vapor cavity 30 is formed and not while electrode 16 is indirect contact with electrolyte 14. The reflection will oscillate withinline 58 with a period determined by its length, and will form a highfrequency (several MHz) modulation.

FIG. 7 illustrates the effect of line 58 on minipulses 50 in a pulse 34.The first set of minipulses 50 does not experience any changes becauseat this time the output impedance is still low (vapor cavity 30 not yetformed). Once vapor cavity 30 is formed reflection occurs andmicropulses 60 are generated. As a result, each minipulse 50 gives riseto a series of micropulses 60. The length of line 58 is selected suchthat micropulses 60 have a duration in the range between 0.1 and 1 μs.The voltage of micropulses 60 is twice as high as that of minipulse 50.This doubling in voltage of micropulses 60 is beneficial because it aidsin ionizing gas 32 to form plasma 28 more rapidly and depositing moreenergy in plasma 28 than it was possible with minipulse 50 at the lowerconstant voltage level. That is because energy deposition increases asthe square of the voltage and only linearly with the amount of time thevoltage is applied. Hence, although micropulses 60 are applied atelectrode 16 only about half the time of a minipulse 50, their doubledvoltage raises the energy deposition by a factor of four.

In accordance with another solution an increase in the rate ofionization of gas 32 is achieved by adding a ballast resistor 62 inseries with the load, as shown in dashed lines in FIG. 1. The resistanceof resistor 62 (R_(ballast)) is selected to be higher than the impedanceof the discharge in electrolyte 14 (R_(electrolyte)) but lower than inthe ionized vapor or gas 32. As a result, the heating of electrolyte 14before evaporation will proceed at a lower voltage U_(low):

U _(low) =U (1+R _(ballast) /R _(electrolyte)).

The reduced voltage will slow the boiling and cause formation of thinnervapor cavity 30. After evaporation the impedance will greatly increase,resulting in an increase of the discharge voltage to a high valueU_(high):

U _(high) =U (1+R _(ballast) /R _(vapor)).

At this high voltage ionization of gas 32 will proceed rapidly.Specifically, when cutting portion 18 has a diameter of 50 μm and itslength L=1 mm the impedance of the discharge in saline 14 is about 500Ohms, while in plasma 28 it is about 6 KOhms. Thus, for example, aballast resistor of 1 kOhms will provide output voltages of U_(low) =U/3and U_(high) =U/1.17, respectively. The lower limit to the voltageapplied during the heating phase is set by how much the duration ofminipulses 50 and pulses 34 can be increased without unacceptablethermal damage to tissue 12 is caused by increased heat diffusion.

In yet another embodiment the method of invention is adaptedspecifically for cutting biological tissue 12 containing muscle tissue12A and nerve tissue 12B. It is known that electric excitation of nervetissue 12B leads to contractions in muscle tissue 12A. In order to avoidcontraction of muscle tissue 12A and reduce the risk of electroporationof adjacent tissue the method of invention calls for limiting andpreferably stopping any charge transfer to tissue 12. This may beachieved by using minipulses 50 of alternating positive and negativepolarities, as illustrated in FIG. 8. Low impedance line 58 can also beused to generate micropulses 60 when vapor cavity 30 is formed.

The polarities may be set by the voltage source of unit 24 in accordancewith well-known electronics techniques. In the present embodiment thealternating polarities can be produced by a separating capacitor (notshown). The discharge time constant of the RC circuit, where R is theresistance of the discharge, should not exceed the excitation time ofnerve cells in nerve tissue 12B at the applied voltage level. A personskilled in the art will appreciate that exact RC time constant will haveto be adjusted on a case-by-case basis. In general, however,contractions of muscle tissue 12A will be prevented at a voltage levelof 500 Volts if the discharge time does not exceed 1 μs. When cuttingportion 18 has a diameter of 50 μm and length L=1 mm the electricalimpedance is about 500 Ohms, and hence the capacitance of capacitorshould not exceed 2 nF. It should be noted that in addition topreventing muscular contractions, alternating polarity of minipulses 50reduces the effect of electroporation, as compared to direct current(DC) (only positive or only negative voltage) pulses.

Various alternatives can be introduced to the apparatus of inventiondepending on the material being cut and the type of cut required. Forexample, in FIG. 9 a cutting electrode 80 of an apparatus analogous toapparatus 10 is used for performing a circular incision 84 in a material82. The return electrode and liquid conducting medium are not shown inthis drawing. Material 82 is a thin sheet of plastic or biologicalmaterial. When used for performing biopsy, a cylindrical biopsy can beeasily obtained in this manner without bleeding.

FIG. 10 illustrates a cutting electrode 90 having two bends 92, 94 toform a U-shaped electrode. The return electrode and liquid conductingmedium are not shown in this drawing. Cutting electrode 90 is used forremoving a large amount of a material 96 with a single cut. U-shapedcutting electrode 90 can be used to minimize the damage to tissue inelectrosurgery and to maximize the lifetime of cutting electrode 90. Inan alternative version a cutting electrode with a single bend can beused to make and L-shaped cutting electrode. In general, bends atvarious angles can be introduced to cutting electrode to perform anydesired type of cut, to approach tissue at various angles and tomanipulate the tissue before and during the cutting.

FIG. 11A illustrates a portion of yet another apparatus 100 having amechanism 102 for advancing a cutting electrode 104. In this embodimentcutting electrode 104 is a wire electrode. Return electrode 106 is inthe form of two capillaries through which wire electrode 104 isthreaded. Capillaries 106 can be used for delivering an electrolyteand/or aspiring fluids during electrosurgery, i.e., capillaries 106 canbe used for irrigation and suction. Cutting electrode 104 forms a loop108 for cutting tissue in accordance with the method of the invention.Mechanism 102 allows the user to refresh cutting electrode as neededduring operation. Exposure time of wire electrode 104 outsidecapillaries 106 should be smaller than its erosion lifetime. It shouldbe noted that mechanism 102 can be used in other embodiments for bothadvancing and retracting the cutting electrode as necessary to maximizeits lifetime and/or retract an eroded electrode.

FIG. 11B illustrates a portion of an apparatus 110 using a wireelectrode 112 threaded through capillaries 114. Capillaries 114 servethe dual function of return electrode and channels for delivering andaspiring fluids during operation. Apparatus 110 can be used as a framesaw, as required in electrosurgical applications. FIG. 11C illustrates aportion of still another apparatus 120 functioning as a stationaryscissors for both lifting and cutting of tissue. Apparatus 120 has acutting electrode 122 in the form of a wire threaded through twocapillaries 124 functioning as the return electrode. Mechanism 102allows the user to refresh cutting electrode as needed during operation.Exposure time of wire electrode 112 outside capillaries 114 should besmaller than its erosion lifetime. A projection 126 is used for liftingof tissue. Both apparatus 110 and apparatus 120 are operated inaccordance with the method of the invention.

FIG. 12 illustrates a portion of an apparatus 130 specifically designedfor capsulotomy. An electrosurgical probe 132 for cap sulotomy has ashape similar to the mechanical tools used for capsulotomy in order tomake its application easy and convenient for surgeons who are used tosuch mechanical tools (comparison is shown in the top photograph). Probe132 has an insulator 134 with external diameter varying between 0.1 and1 mm, which has a bent tip 136 at the end. A cutting electrode 138 witha diameter varying between 10 to 200 microns protrudes from insulator134 by a distance varying between 20 microns to 1 mm. A return electrode140 can be either a concentric needle or an external electrode attachedto the eye or somewhere else to the body of the patient. Apparatus 130protects the tissue located above the lens capsule (cornea and iris)(not shown) from accidental contact with cutting electrode 138 thusensuring its safe use during capsulotomy.

Duty Cycle

The pulse waveforms described herein operate at a low duty cycle,typically less than or equal to 0.1 or 10%. In some variations, the lowduty-cycle pulse waveform for cutting includes repeated bursts ofminipulses, where each burst of minipulses is separated by an interburst(or interpulse) interval. Each burst of minipulses allows a completecycle of action of the cutting electrode. Thus, each burst of minipulsesmay perform a complete cycle of vaporization, ionization, andtermination, necessary for cutting by the electrode, as described above.

Duty cycle is generally understood as the ratio of “on” time to “off”time for an electronic component or signal. This ratio (which may rangefrom 0 to 1), may also be expressed as a percent (from 0% to 100%). In apulsed waveform (e.g., the pulse regime), the duty cycle may be definedas the ratio of (a) the sum of all pulse durations during a specifiedperiod of continuous operation to (b) the total specified period ofoperation. The methods and devices described herein describeplasma-mediated thermo-electric ablation having a low (e.g., less than10%, less than 5%, less than 2%, less than 1% and less than 0.1%) dutycycle during cutting using the cutting pulse waveform. In operation,this low duty cycle may result in substantially lower damage to adjacenttissues, since the total energy applied to the tissue is much lower thanhigher duty-cycle methods, and is applied in a much more precise mannerthan other electrosurgical techniques. As described in more detailbelow, the lower energy applied may be seen when observing the heat ofthe cutting electrode during cutting.

In addition to the low duty-cycle pulse waveforms previously described,FIGS. 13A-13E illustrate additional low duty-cycle pulse waveforms thatmay be used to cut tissue as described herein. The general pulsewaveform shown in FIG. 13A is similar to the pulse waveform shown anddescribed above in FIG. 2. In FIG. 13A, the pulse waveform consists ofrepeated bursts 1301 of minipulses. As mentioned above, a burst ofminipulses may also be referred to as a pulse or a burst. Each burst ofminipulses has a minipulse duration 1303, and is separated from the nextburst of minipulses by an interburst interval 1305. Thus, the bursts ofminipulses are repeated at a repetition rate (“rep rate”). Table 1,below, illustrates examples of values for the rep rate, and minipulseburst duration for various low duty-cycle pulse waveforms. For example,a pulse waveform for a low-duty cycle of less than about 10% duty cyclemay have a rep rate of between about 10 Hz and 1 KHz (e.g., aninterburst interval of approximately 1 ms to 100 ms), and a minipulseburst duration (pulse duration) of between about 10 μs and 100 μs. Insome variations, a pulse waveform for a low-duty cycle of less thanabout 10% duty cycle may have a rep rate of between about 10 Hz and 500Hz and a minipulse burst duration of between about 5 μs and about 200 μsor between about 10 μs and 200 μs. An exemplary pulse waveform for alow-duty cycle of less than about 5% duty cycle may have a rep rate ofbetween about 10 Hz and 500 Hz (e.g., an interburst interval ofapproximately 2 ms to 100 ms), and a minipulse burst duration (pulseduration) of between about 10 μs and 100 μs. An exemplary pulse waveformfor a low-duty cycle of less than about 2.5% duty cycle may have a reprate of between about 10 Hz and 250 Hz (e.g., an interburst interval ofapproximately 1 ms to 100 ms), and a minipulse burst duration (pulseduration) of between about 10 μs and 100 μs.

FIG. 13B shows a magnified view of the burst of minipulses 1301 within alow duty-cycle pulse waveform. The burst of minipulses includes aplurality of minipulses 1315. In FIG. 13B, the minipulses are eachidentically shaped. As previously described, the minipulses may havedifferent pulse shapes. Each of the minipulses shown in FIG. 13B arebipolar minipulses, so that each minipulses have a positive and anegative voltage component (+V and −V). In some variations, theminipulses are not bipolar, but are monopolar, or have alternatingpolarities. FIG. 13C illustrates a burst of pulses 1316 that are notbipolar. The minipulses in FIG. 13B are also not separated by anintraburst interval (e.g., the time between minipulses is 0s). Thus, theburst of minipulses in FIG. 13B is a continuous burst of minipulses,within the burst. In some variations, there is an intraburst intervalthat is non-zero. Fore example, the intraburst interval may be between10 ns and 50 μs.

The duration of each minipulse within the burst of minipulses may beselected from an appropriate range, as previously described. Forexample, in some variations, the duration of the minipulse is betweenabout 10 ns and about 10 μs. Table 1 also gives some exemplary values ofminipulse duration, duration between minipulses (intraburst interval)and the number of minipulses. The minipulses within each minipulse burstmay have any appropriate voltage, as described above, particularly forthe initiation and maintenance of plasma. For example, the voltage maybe between about ±600 V, or between about ±500 V, or between about ±400V.

FIG. 13D shows an example of a pulse waveform having a duty cycle ofless than 1% (e.g., 0.01%). In this example, the minipulse burstduration 1317 is approximately 10 μs, and the rep rate 1319 isapproximately 10 Hz. The minipulses within the burst are bipolar, andare continuous. FIG. 13E is another example of a low duty-cycle pulsewaveform. In this example, the duty cycle is approximately 5%. The burstduration 1321 is approximately 100 μs, and the rep rate is approximately500 Hz. Other exemplary pulse waveform parameters are shown in Table 1.

TABLE 1 Exemplary Low duty-cycle pulse waveform parameters MinipulseMini- Burst pulse Interval Duty Repetition burst Minipulse Number ofBetween Cycle rate duration Duration Minipulses Minipulses   ~10% 1 KHz100 μs  10 ns 5000 10 ns   ~5% 500 Hz 100 μs  100 ns  1000 0 s  ~2.5%300 Hz 83 μs 10 ns 8300 0 s   ~1% 200 Hz 50 μs 250 ns  200 0 s  ~0.5% 50Hz 100 μs  50 ns 1000 50 ns ~0.25% 250 Hz 10 μs 10 ns 1000 0 s  ~0.1% 20Hz 50 μs 10 ns 2500 10 ns ~0.01% 10 Hz 10 μs 20 ns 500 0 s

A low duty-cycle cutting pulse waveform may be used for cutting tissue,particular biological tissue. In some variations, the low duty-cyclecutting pulse waveform has a duty cycle of less than 2.5% (e.g., betweenabout 2.5% and 0.1%). For example, relatively dry skin may be cut usingan elongated electrode having an exposed edge (or a wire electrode, asdescribed above) at a low duty-cycle of approximately 2% (e g ,minipulse burst duration of 75 μs, minipulse duration of 250 ns, a reprate of 200 Hz, and a voltage of approximately ±425 V).

As described above, there is a relationship between the length or sizeof the electrode and the ability to cut at low duty-cycle. For example,longer or wider electrodes may require more energy to initiate andmaintain plasma for cutting. The examples provided herein may be usedwith any relatively long (e.g., less than 5 mm long) electrode. Inoperation, the duty-cycle (or relevant parameters affecting theduty-cycle such as the rep rate of the burst of minipulses, the durationof the burst of minipulses, the duration of the minipulses, the voltageof the minipulses, the intraburst interval of the minipulses, or thenumber of minipulses) may be adjusted to affect efficient cutting. Forexample, the duty cycle or relevant parameters may be selected fromwithin a preset range, or from within a table of preset values.

In some variations, the pulse generator and/or voltage control unitallows selection and control of the pulse waveform applied. The pulsewaveform applied may be matched to information provided to the system.For example, the cutting electrode may communicate with the voltagecontrol unit or pulse generator (generally referred to as the powersupply), indicating that the exposed portion of the cutting electrodehas a particular length and/or width, and/or shape. The pulse waveformapplied may be selected, in part, based on this information.

The voltage control unit and/or power supply may also include user inputto select the low-duty cycle pulse waveform. In some variations, thevoltage control unit may include control circuitry for calculating ordetermining duty-cycle, either directly or by approximation. In somevariations, the voltage control unit may have a regulator or governorthat prevents the duty-cycle for a cutting pulse waveform to exceed 10%,5%, 2.5%, etc. Thus, only low duty-cycle pulse waveforms may be appliedfor cutting. The pulse waveform may be selected by allowing selection ofsome or any of the parameters described. For example, the duty-cycle forcutting may be selected directly. In some variations, the pulse durationis selectable. The rep rate of the burst of minipulses is selectable(within a range, e.g., 10 Hz-500 Hz). In some variations, the durationof the burst of minipulses is selectable within a range, e.g.,10μs-100μs). In some variations, the duration of the minipulses isselectable within a range, e.g., 10 ns-100 ns). In some variation, thevoltage of the minipulses is selectable within a range, e.g., ±600V). Insome variations, the intraburst interval of the minipulses is selectablewithin a range, e.g., 0s-100 ns. In some variations, the number ofminipulses within a burst is selectable within a range, e.g., 100-10000.The voltage controller may include any combination of these controls toachieve the cutting pulse waveform.

One consequence of the low-duty cycle pulse waveform is that the averagetemperature of the cutting electrode (and particularly the temperatureof the exposed, or uninsulated cutting region) is substantially lowerthan cutting electrodes driven by non-low duty-cycle pulse waveforms, ordriven by continuous RF energy. Examples of this are shown in FIG.14A-14C, which show thermal images of electrodes driven by differentstimulation regimes (including pulse waveforms). The temperature of theregion imaged in FIGS. 14A-14C roughly corresponds to the lightintensity in the images.

FIG. 14A shows a cutting electrode having a curved edge that isuninsulated during the application of a cutting pulse waveform having aduty cycle of approximately 1%. By examining thermal images such as thisone, it was determined that the average temperature over the cuttingedge of the cutting electrode during application of a low duty-cyclepulse waveform was approximately 37° C. FIG. 14B shows another cuttingelectrode (e.g., a ValleyLab™ “electrosurgical pencil”) driven by acontinuous RF waveform at relatively low power (e.g., 30 W). As thethermal image in FIG. 14B illustrates, the average temperature over thecutting portion of the electrode is much higher, and in this example wasgreater than 300° C. FIG. 14C shows another example of an electrode(Arthrocare™ TurboVac at set point 8, approximately 300 Volts) in whichthe average temperature of the electrode is 110° C. In FIGS. 14A-14C,the ambient temperature of the tissue was below 37° C. (e.g., the tissuewas chilled).

Although the average temperature of the cutting electrode during theapplication of a low duty-cycle pulse waveform may be significantly lessthan 100° C., the instantaneous temperature at the cutting edge may bemuch greater than 100° C., as described above. Under the pulse waveformsdescried herein, formation of plasma (e.g., formation of a vapor cavity,ionization, etc.) may result in very high temperatures (much greaterthan 100° C.) during a portion of the minipulse burst. However, at lowduty cycles the pulse waveform comprises a burst of minipulses separatedby an interburst interval (rep rate) that is sufficiently long to allowthe temperature at the edge of the electrode to relax back down,preventing sustained heating of the electrode while allowing cuttingvia. the plasma.

Thus, in some variations, the pulse waveform selected to be applied maybe selected based on the average temperature. For example,thermo-electrical cutting of biological tissue may be performed byapplying a low duty-cycle pulse waveform to a cutting electrode whereinthe cutting electrode has an average temperature during application ofthe pulse waveform of less than about 50° C. (when the temperature ofthe tissue is initially at approximately 37° C. or cooler). The peaktemperature during application of this pulse waveform may be greaterthan 100° C. In some variations, the average temperature duringapplication of the pulse waveform is less than about 40° C.

The apparatus and method of the invention ensure efficient thermalablation at low power levels, e.g., ranging down to 10 mW by overheatingand evaporation. Devices built in accordance with the invention can beused for cutting various types of materials including biological tissuewhile minimizing the damage zone and heat accumulation in the materialbeing cut as well as the surroundings and the hand piece. The voltagesnecessary for producing the plasma are reduced significantly incomparison to prior art devices. Because of such power efficiency andlow thermal damage the apparatus of invention and method for operatingit can be adapted to numerous applications in surgery on very sensitiveorgans, such as the eye. For example, the apparatus of invention can beused for: (a) dissection of membranes and cutting retina invitreoretinal surgery, (b) capsulotomy, (c) lensectomy, (d) iridectomy,(e) trabeculectomy.

As mentioned briefly above, the apparatus and methods described hereinmay also be used for some blood control or hemostatis. For example, inaddition to (or instead of) cutting or ablating tissue, the electrode(cutting electrode) may be used to prevent blood loss by not onlycauterizing blood vessels, but also constricting them with very littlethermal damage. Thus the described methods may be methods of preventingblood loss by applying a pulse waveform having a duty-cycle of less than10% to a cutting electrode, as described herein.

FIGS. 15A and 15B are sections through tissue cut using a cuttingelectrode and the low duty-cycle pulse waveform described above, andillustrates the hematostatic effect of the low duty-cycle cutting onvessels adjacent to the cut. In particular, FIGS. 15A and 15B show CAM(chick chorioallantoic membrane) tissue that was cut by a low duty-cyclewaveform (with a 300 Hz rep rate, having bursts of minipulses ofapproximately 200 minipulses at ±450V). In FIG. 15A, the tissue has beencut 1501, so that at least one blood vessel 1503 was been severed. FIG.15B shows a magnified view of this blood vessel 1503. The severed bloodvessel is substantially constricted for a short distance from the regionof the cut 1501, as indicated by the space between the arrows 1508.Thus, the internal lumen of the blood vessel has been reduced to adiameter of less than about 15 μm. Further along the length of thevessel away from the cut 1501, the blood vessel has a more normaldiameter, 1510 (e.g., greater than 40 μm). This vasoconstriction resultsin a reduction or elimination of bleeding from the cut vessel, andappears in both arteries and veins cut by the low duty-cycle cuttingwaveform.

A similar hematostatic effect of electrical energy applied to tissue isdescribed in PCT application WO PCT/U.S. 2005/033856, filed Sep. 20,2005 (titled “METHODS AND DEVICES FOR THE NON-THERMAL,ELECTRICALLY-INDUCED CLOSURE OF BLOOD VESSELS”), herein incorporated byreference in its entirety. This effect of low duty-cycle cutting isparticularly surprising and unexpected, and may provide the cuttingmethods and devices described herein with unexpected advantages. Theeffect does not appear to be dependent on thermal coagulation. Asdescribed above, the average temperature of a cutting electrodestimulated by a low duty-cycle pulse waveform is relatively low. Thus,there may be less thermal damage. Although the biological mechanismbehind the hemostatic effect is not completely understood, it may berelated to calcium ion channels, since calcium channel agonists may atleast partly inhibit this effect.

Thus, when cutting tissue using any of the cutting electrodes describedabove using any of the low duty-cycle pulse waveforms described,bleeding may be substantially reduced by the non-thermal hematostatis(or “non-thermal coagulation”). FIG. 16A is a graph comparing bleedingfrom a cutting electrode using a low duty-cycle pulse waveform to ascalpel (blade), and to a high voltage, high duty-cycle device control(Control 1) that cauterizes as it cuts. In this example, porcine tissuewas cut (e.g., porcine skin and muscle tissue) with a scalpel, a controlelectrosurgical device operating under continuous RF stimulation (100%duty cycle), and with a cutting electrode operating on a low duty-cycle.The control 1 (100% duty cycle) device was a ValleyLab™ “electrosurgicalpencil” operating at 40 W in the fulguration mode. The low duty-cyclewaveform had a rep rate 200 Hz with burst of approximately 200minipulses per burst, and a voltage of ±425V. As shown in FIG. 16A, thecutting electrode using a low duty-cycle pulse waveform bleeds less thana comparable cut made by a scalpel. In this example the low duty-cyclecut bled approximately 71.3% less bleeding than scalpel (N=3, p=0.003).Although cauterization using a traditional electrosurgical device (e.g.,the control 1 device), thermal cauterization may also result insubstantially more tissue damage, and may promote scar formation. FIG.16B shows a graph illustrating wound strength for cuts similar made withthe low duty-cycle cutting electrode 1610 (diamonds), a thermallycauterizing cutting electrode 1612 (squares), and a scalpel 1614(triangles).

In FIG. 16B, wound strength is measured at different times after a cutwas made. In this example, comparable cuts were made in porcine skintissue using either a cutting electrode under a low duty-cycle pulsewaveform, a scalpel, or a cauterizing electrosurgical device (driven bya continuous RF waveform). Wound strength was measured 1 week after acut, 2 weeks after a cut, 3 weeks after a cut, and six weeks after acut. Cuts were approximately 2 cm long. Tissue strength was determinedusing a tensile tester applied across the cut tissue after it had healedfor the indicated amount of time, and force was applied untilseparation. As FIG. 16B shows, the wound strength of the cut formed bythe low duty-cycle cut is comparable to that formed by the scalpel, andboth are slightly stronger than the thermally cauterized cut at eachtime point measured (thermal cuts are approximately 60% less strong thanscalpel and low duty-cycle cuts). Although such results may varydepending on the size and location of the cut made, as well as the typeof tissue, such results are highly suggestive that low duty-cyclecutting by a cutting electrode may both inhibit bleeding and may promotehealing

The above detailed description is provided to illustrate exemplaryembodiments and is not intended to be limiting. For example, any of thefeatures of an embodiment may be combined with some or all of thefeatures of other embodiments. It will be apparent to those skilled inthe art that numerous modifications and variations within the scope ofthe present invention are possible. Throughout this description,particular examples have been discussed, including descriptions of howthese examples may address certain disadvantages in related art.However, this discussion is not meant to restrict the various examplesto methods and/or systems that actually address or solve thedisadvantages. Accordingly, the present invention is defined by theappended claims and should not be limited by the description herein.

1. (canceled)
 2. The method of claim 20, wherein applying the cuttingwaveform comprises applying a plurality of pulses having a duty cycle ofless than 10%.
 3. The method of claim 20, wherein applying the cuttingwaveform comprises applying a plurality of pulses having a duty cycle ofless than 5%
 4. The method of claim 20, wherein applying the cuttingwaveform comprises applying a plurality of pulses having a duty cycle ofbetween about 2.5% and about 0.01%.
 5. The method of claim 20, whereinapplying the cutting waveform comprises applying a plurality of pulseswherein each pulse comprises a burst of minipulses.
 6. The method ofclaim 5, wherein the minipulses within the burst of minipulses comprisebipolar minipulses.
 7. The method of claim 5, wherein the duration ofeach minipulse within the burst of minipulses is between about 10 ns andabout 100 μs.
 8. The method of claim 5, wherein the minipulses withineach burst of minipulses are continuously applied.
 9. The method ofclaim 20, wherein applying the low duty cycle cutting waveform comprisesapplying a plurality of bursts of minipulses having an interburstrepetition rate of between about 10 Hz and 500 Hz, and a minipulse burstduration of between about 5 μs and about 200 μs.
 10. The method of claim20, wherein applying the low duty cycle cutting waveform comprisesapplying a plurality of pulses having a voltage of between about −500 Vand about +500 V. 11-19. (canceled)
 20. A method for thermo electricalcutting of biological tissue comprising: applying a low duty cycle pulsewaveform to a cutting electrode, wherein the cutting electrode has apeak temperature during application of the pulse waveform of greaterthan 100° C. and an average temperature during application of the pulsewaveform of less than about 100° C.; and cutting the tissue with thecutting electrode during application of the low duty cycle pulsewaveform.
 21. The method of claim 20, wherein the average temperature ofthe cutting electrode during application of the pulse waveform is lessthan about 40° C.
 22. A method of simultaneously cutting and hemostasisof a biological tissue, the method comprising: contacting a biologicaltissue with a cutting electrode, applying a low duty cycle pulsewaveform to the cutting electrode, wherein the cutting electrode has apeak temperature during application of the pulse waveform of greaterthan 100° C. and an average temperature during application of the pulsewaveform of less than about 100° C.; and cutting the tissue with thecutting electrode during application of the low duty cycle pulsewaveform while at least partially constricting the blood vesselsadjacent to the cut tissue.
 23. The method of claim 22, wherein applyingthe low duty cycle cutting waveform comprises applying a plurality ofpulses having a duty cycle of less than 10%.
 24. The method of claim 22,wherein applying the low duty cycle cutting waveform comprises applyinga plurality of pulses having a duty cycle of less than 5%.
 25. Themethod of claim 22, wherein applying the low duty cycle cutting waveformcomprises applying a plurality of pulses wherein each pulse comprises aburst of minipulses.
 26. The method of claim 25, wherein the minipulseswithin the burst of minipulses comprise bipolar minipulses.
 27. Themethod of claim 25, wherein the duration of each minipulse within theburst of minipulses is between about 10 ns and about 100 μs.
 28. Themethod of claim 25, wherein the minipulses within each burst ofminipulses are continuously applied.
 29. The method of claim 22, whereinapplying the low duty cycle cutting waveform comprises applying aplurality of bursts of minipulses having an interburst repetition rateof between about 10 Hz and 500 Hz, and a minipulse burst duration ofbetween about 5 μs and about 200 μs.
 30. The method of claim 22, whereinapplying the low duty cycle cutting waveform comprises applying aplurality of pulses having a voltage of between about −500 V and about+500 V.
 31. The method of claim 22, wherein the average temperatureduring application of the pulse waveform is less than about 40° C. 32.(canceled)